Along with the house, they include the park bench where the Skripals were found collapsed, the Zizzi restaurant where they had dined beforehand, and the home of Detective Sergeant Nick Bailey, who was exposed to the agent.

Lt Gen Urch, commander home command and standing joint commander, said it had been “the longest running” operation of its kind on British soil.

He said: “Novichok is probably one of the most dangerous and most challenging chemicals in existence today and you don’t need very much of it and it’s highly spreadable.”

An estimated 600 to 800 specially trained military personnel – including the chemical, biological, radiological and nuclear regiment – were involved in the clean-up, named Operation Morlop.

Around 190 worked at any one time and there were 250 “deliberate operations” to cross into and decontaminate danger areas.